The present invention is a method for the fractionation of de-asphalted oil of vacuum resid using preparative High Performance Liquid Chromatographic (HPLC) separations.
The petroleum vacuum resid de-asphalted oils (DAOs), boiling above ˜1000° F., are too complex to be analyzed as such for detailed compositional information even with the most modern analytical techniques/instruments. It is, therefore, essential to fractionate them into several sub-fractions so that the detailed/extensive compositional analyses can be performed using high resolution mass spectrometry and other analytical techniques.
Extensive documentation exists in the literature for the class separation of hydrocarbons and heteroatoms, such as nitrogen, oxygen and sulfur containing petroleum compounds in various petroleum streams (1-5). Silica gel or alumina have been the most common substrates for the LC fractionation of heavy petroleum streams in to total saturates (paraffins plus cycloparaffins also called naphthenes), total aromatics and total polars. In some cases, amino and/or cyano functional groups bonded to silica gel have been used for similar saturates, aromatics and polars separations.
The separation of sulfur compounds, such as sulfides, is more difficult to accomplish with conventional LC substrates. Because of the unique interaction of sulfides with certain metallic ions, ligand exchange chromatography has been documented. The most common metallic ions used have included silver (6), palladium (3), mercury (7) and copper (8). In general, these metallic ions are deposited from their salts onto substrates like silica gel (6) or week-ion exchange substrates (8). To the best of our knowledge, the use of strong-ion exchange substrate such as alkyl- or aryl-sulfonic functional groups bonded to silica gel and containing silver ions for the isolation of sulfides has not been documented in the literature.
The use of ligand exchange with sulfonic functional groups containing silver ions has been demonstrated for the separation of total aromatics from total saturates (9, 10) for lube basestocks. The advantage of such substrate for total saturates and total aromatics is the enhanced separation imparted by the silver ion.
The separation of aromatics into subclasses consisting of mono-, di-, tri- and or tetra+ aromatic rings separations are best accomplished using several solid supports e.g. silica gel, bonded amino (3), amino-cyano (3) and dinitroanilinopropyl (DNAP) (3). The advantage of using bonded substrates is that the separation column(s) are re-useable for many separations.
The complexity of aromatic hydrocarbons types increases significantly with increasing molecular weight and/or boiling point of the petroleum fractions. As a result, the separation of the aromatic group into individual ring numbers (i.e. mono-, di- etc) is challenging as compound class overlap may occur, particularly with sterically hindering alkylgroups substituted on the individual aromatic rings. In addition, the presence of saturated cyclic rings on the aromatic rings may lead to longer retention on the column substrates and overlap with the next aromatic ring class. To enhance the resolution among the aromatic ring classes of compounds, cryogenic HPLC separation at temperatures of <0° C. are more effective. The current literature is void in such low temperature operating conditions when applied to the separation of heavy petroleum fractions.
The present invention presents a relatively efficient separation scheme for heavy petroleum fractions. It combined in a single step a separation of total saturates, total aromatics, polars and sulfides. A second offline separation fractionates the total aromatics from the first step into aromatic rings using a cryogenic separation at temperatures substantially lower than 0° C. The cryogenic separation enhances the resolution among the aromatics rings.